Central command and a reflex originating in working skeletal muscles are two possible control mechanisms responsible for the cardiorespiratory responses to exercise. Even though these two hypotheses have been studied separately, little is known about interactions between these two control mechanisms. Therefore the proposed project will attempt to locate central neural sites in cats that are involved in both central command and peripheral feedback. One series of experiments will be performed in anesthetized cats. First, the cardiorespiratory responses to stimulation in the subthalamic fields of Forel, a maneuver thought to activate central command, will be determined before and after lesions are placed in a medullary site (the lateral reticular nucleus) believed to be associated with the reflex cardiovascular responses to muscular contraction. Next, cardiovascular and ventilatory responses to muscular contraction induced by ventral root stimulation will be measured before and after ablation of the fields of Forel. Finally, fields of Forel stimulation and ventral root stimulation will be performed separately before and after lesioning a cerebellar area (the fastigial nucleus) involved in the cardiorespiratory responses to exercise. In a second series of experiments, cats will be instrumented for chronic cardiovascular measurements and then trained to perform isometric exercise. These conscious cats will be studied before and after making lesions identical to those made in the anesthetized cats. In a final series of experiments, microelectrode recordings will be made in anesthetized cats to establish if the fields of Forel, fastigial nucleus or the lateral reticular nucleus have appropriate neural connections for integration of central command and feedback from exercising muscles. It is hoped that these experiments will identify central neural sites that integrate descending central command with peripheral feedback to provide the appropriate responses to exercise. This knowledge may be useful to clinicians having patients with neurological deficits causing abnormal responses to exercise.